select — Waiting for I/O completion等待I/O完成

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This module provides access to the select() and poll() functions available in most operating systems, devpoll() available on Solaris and derivatives, epoll() available on Linux 2.5+ and kqueue() available on most BSD. 此模块提供对大多数操作系统中可用的select()和poll()函数的访问，Solaris和衍生产品上可用的devpoll()，Linux 2.5+上可用的epoll()，以及大多数BSD上可用的kqueue()。Note that on Windows, it only works for sockets; on other operating systems, it also works for other file types (in particular, on Unix, it works on pipes). 注意，在Windows上，它只适用于套接字；在其他操作系统上，它也适用于其他文件类型（特别是在Unix上，它适用于管道）。It cannot be used on regular files to determine whether a file has grown since it was last read.不能在常规文件上使用它来确定文件自上次读取后是否已增长。

Note

The selectors module allows high-level and efficient I/O multiplexing, built upon the select module primitives. selectors模块允许基于select模块原语构建的高级高效I/O复用。Users are encouraged to use the selectors module instead, unless they want precise control over the OS-level primitives used.鼓励用户改用selectors模块，除非他们希望对所使用的OS级原语进行精确控制。

The module defines the following:该模块定义了以下内容：

exceptionselect.error

A deprecated alias of OSError.OSError的已弃用别名。

Changed in version 3.3:版本3.3中更改： Following PEP 3151, this class was made an alias of OSError.继PEP 3151之后，该类被称为OSError的别名。

select.devpoll()

(Only supported on Solaris and derivatives.) （仅在Solaris和衍生产品上受支持。）Returns a /dev/poll polling object; see section /dev/poll Polling Objects below for the methods supported by devpoll objects.返回/dev/poll轮询对象；有关devpoll对象支持的方法，请参阅下面的/dev/poll轮询对象部分。

devpoll() objects are linked to the number of file descriptors allowed at the time of instantiation. devpoll()对象链接到实例化时允许的文件描述符数量。If your program reduces this value, devpoll() will fail. 如果程序减少了这个值，devpoll()将失败。If your program increases this value, devpoll() may return an incomplete list of active file descriptors.如果程序增加这个值，devpoll()可能会返回一个不完整的活动文件描述符列表。

The new file descriptor is non-inheritable.新文件描述符不可继承。

New in version 3.3.版本3.3中新增。

Changed in version 3.4:版本3.4中更改： The new file descriptor is now non-inheritable.新文件描述符现在不可继承。

select.epoll(sizehint=- 1, flags=0)

(Only supported on Linux 2.5.44 and newer.) （仅在Linux 2.5.44和更高版本上受支持。）Return an edge polling object, which can be used as Edge or Level Triggered interface for I/O events.返回边缘轮询对象，该对象可用作I/O事件的边缘或级别触发接口。

sizehint informs epoll about the expected number of events to be registered. sizehint通知epoll要注册的预期事件数。It must be positive, or -1 to use the default. 它必须为正值，或-1才能使用默认值。It is only used on older systems where epoll_create1() is not available; otherwise it has no effect (though its value is still checked).它仅用于epoll_create1()不可用的旧系统；否则它没有效果（尽管它的值仍然被检查）。

flags is deprecated and completely ignored. However, when supplied, its value must be 0 or select.EPOLL_CLOEXEC, otherwise OSError is raised.

See the Edge and Level Trigger Polling (epoll) Objects section below for the methods supported by epolling objects.请参阅下面的边缘和级别触发器轮询（epoll）对象部分，了解epoll对象支持的方法。

epoll objects support the context management protocol: when used in a with statement, the new file descriptor is automatically closed at the end of the block.

The new file descriptor is non-inheritable.

Changed in version 3.3:版本3.3中更改： Added the flags parameter.

Changed in version 3.4:版本3.4中更改： Support for the with statement was added. The new file descriptor is now non-inheritable.

Deprecated since version 3.4: The flags parameter. select.EPOLL_CLOEXEC is used by default now. Use os.set_inheritable() to make the file descriptor inheritable.

select.poll()

(Not supported by all operating systems.) （并非所有操作系统都支持。）Returns a polling object, which supports registering and unregistering file descriptors, and then polling them for I/O events; see section Polling Objects below for the methods supported by polling objects.返回一个轮询对象，该对象支持注册和注销文件描述符，然后轮询I/O事件；有关轮询对象支持的方法，请参阅下面的轮询对象部分。

select.kqueue()

(Only supported on BSD.) （仅在BSD上受支持。）Returns a kernel queue object; see section Kqueue Objects below for the methods supported by kqueue objects.返回内核Kqueue对象；有关Kqueue对象支持的方法，请参阅下面的Kqueue对象部分。

The new file descriptor is non-inheritable.新文件描述符不可继承。

Changed in version 3.4:版本3.4中更改： The new file descriptor is now non-inheritable.新文件描述符现在不可继承。

select.kevent(ident, filter=KQ_FILTER_READ, flags=KQ_EV_ADD, fflags=0, data=0, udata=0)

(Only supported on BSD.) （仅在BSD上受支持。）Returns a kernel event object; see section Kevent Objects below for the methods supported by kevent objects.返回内核事件对象；有关Kevent对象支持的方法，请参阅下面的Kevent对象部分。

select.select(rlist, wlist, xlist[, timeout])

This is a straightforward interface to the Unix select() system call. The first three arguments are iterables of ‘waitable objects’: either integers representing file descriptors or objects with a parameterless method named fileno() returning such an integer:前三个参数是“waitable objects”的可迭代变量：要么是表示文件描述符的整数，要么是名为fileno()的无参数方法返回这样一个整数的对象：

• rlist: wait until ready for reading：等待阅读准备就绪

• wlist: wait until ready for writing：等待，直到准备好写入

• xlist: wait for an “exceptional condition” (see the manual page for what your system considers such a condition)：等待“异常情况”（请参阅手册页面了解系统认为这种情况的原因）

Empty iterables are allowed, but acceptance of three empty iterables is platform-dependent. (It is known to work on Unix but not on Windows.) The optional timeout argument specifies a time-out as a floating point number in seconds. When the timeout argument is omitted the function blocks until at least one file descriptor is ready. 当忽略timeout参数时，该函数将阻塞，直到至少有一个文件描述符就绪。A time-out value of zero specifies a poll and never blocks.超时值为零指定轮询而从不阻止。

The return value is a triple of lists of objects that are ready: subsets of the first three arguments. 返回值是就绪对象列表的三倍：前三个参数的子集。When the time-out is reached without a file descriptor becoming ready, three empty lists are returned.当在文件描述符未就绪的情况下达到超时时，将返回三个空列表。

Among the acceptable object types in the iterables are Python file objects (e.g. sys.stdin, or objects returned by open() or os.popen()), socket objects returned by socket.socket(). 可迭代对象中可接受的对象类型包括Python文件对象（例如sys.stdin，或open()或ospop()返回的对象）、socketsocket()所返回的套接字对象。You may also define a wrapper class yourself, as long as it has an appropriate fileno() method (that really returns a file descriptor, not just a random integer).您也可以自己定义一个包装类，只要它有一个合适的fileno()方法（它确实返回一个文件描述符，而不仅仅是一个随机整数）。

Note

File objects on Windows are not acceptable, but sockets are. Windows上的文件对象是不可接受的，但套接字是可接受的。On Windows, the underlying select() function is provided by the WinSock library, and does not handle file descriptors that don’t originate from WinSock.在Windows上，底层select()函数由WinSock库提供，不处理不源自WinSock的文件描述符。

Changed in version 3.5:版本3.5中更改： The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see PEP 475 for the rationale), instead of raising InterruptedError.当被信号中断时，该函数现在以重新计算的超时重试，除非信号处理程序引发异常（有关原理，请参阅PEP 475），而不是引发InterruptedError。

select.PIPE_BUF

The minimum number of bytes which can be written without blocking to a pipe when the pipe has been reported as ready for writing by select(), poll() or another interface in this module. 当管道被select()、poll()或该模块中的另一个接口报告为准备好写入时，可以在不阻塞的情况下写入管道的最小字节数。This doesn’t apply to other kind of file-like objects such as sockets.这不适用于其他类型的类文件对象，如套接字。

This value is guaranteed by POSIX to be at least 512.POSIX保证该值至少为512。

Availability: Unix

New in version 3.2.版本3.2中新增。

/dev/poll Polling Objects

Solaris and derivatives have /dev/poll. While select() is O(highest file descriptor) and poll() is O(number of file descriptors), /dev/poll is O(active file descriptors).

/dev/poll behaviour is very close to the standard poll() object.

devpoll.close()

Close the file descriptor of the polling object.

New in version 3.4.版本3.4中新增。

devpoll.closed

True if the polling object is closed.如果轮询对象已关闭，则为True。

New in version 3.4.版本3.4中新增。

devpoll.fileno()

Return the file descriptor number of the polling object.

New in version 3.4.版本3.4中新增。

devpoll.register(fd[, eventmask])

Register a file descriptor with the polling object. Future calls to the poll() method will then check whether the file descriptor has any pending I/O events. fd can be either an integer, or an object with a fileno() method that returns an integer. File objects implement fileno(), so they can also be used as the argument.

eventmask is an optional bitmask describing the type of events you want to check for. The constants are the same that with poll() object. The default value is a combination of the constants POLLIN, POLLPRI, and POLLOUT.

Warning

Registering a file descriptor that’s already registered is not an error, but the result is undefined. The appropriate action is to unregister or modify it first. This is an important difference compared with poll().

devpoll.modify(fd[, eventmask])

This method does an unregister() followed by a register(). It is (a bit) more efficient that doing the same explicitly.

devpoll.unregister(fd)

Remove a file descriptor being tracked by a polling object. Just like the register() method, fd can be an integer or an object with a fileno() method that returns an integer.

Attempting to remove a file descriptor that was never registered is safely ignored.

devpoll.poll([timeout])

Polls the set of registered file descriptors, and returns a possibly-empty list containing (fd, event) 2-tuples for the descriptors that have events or errors to report. fd is the file descriptor, and event is a bitmask with bits set for the reported events for that descriptor — POLLIN for waiting input, POLLOUT to indicate that the descriptor can be written to, and so forth. An empty list indicates that the call timed out and no file descriptors had any events to report. If timeout is given, it specifies the length of time in milliseconds which the system will wait for events before returning. If timeout is omitted, -1, or None, the call will block until there is an event for this poll object.

Changed in version 3.5:版本3.5中更改： The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see PEP 475 for the rationale), instead of raising InterruptedError.

Edge and Level Trigger Polling (epoll) Objects

https://linux.die.net/man/4/epoll

eventmask

Constant	Meaning
EPOLLIN	Available for read
EPOLLOUT	Available for write
EPOLLPRI	Urgent data for read
EPOLLERR	Error condition happened on the assoc. fd
EPOLLHUP	Hang up happened on the assoc. fd
EPOLLET	Set Edge Trigger behavior, the default is Level Trigger behavior
EPOLLONESHOT	Set one-shot behavior. After one event is pulled out, the fd is internally disabled
EPOLLEXCLUSIVE	Wake only one epoll object when the associated fd has an event. The default (if this flag is not set) is to wake all epoll objects polling on a fd.
EPOLLRDHUP	Stream socket peer closed connection or shut down writing half of connection.
EPOLLRDNORM	Equivalent to EPOLLIN
EPOLLRDBAND	Priority data band can be read.
EPOLLWRNORM	Equivalent to EPOLLOUT
EPOLLWRBAND	Priority data may be written.
EPOLLMSG	Ignored.

New in version 3.6.版本3.6中新增。EPOLLEXCLUSIVE was added. It’s only supported by Linux Kernel 4.5 or later.

epoll.close()

Close the control file descriptor of the epoll object.

epoll.closed

True if the epoll object is closed.

epoll.fileno()

Return the file descriptor number of the control fd.

epoll.fromfd(fd)

Create an epoll object from a given file descriptor.

epoll.register(fd[, eventmask])

Register a fd descriptor with the epoll object.

epoll.modify(fd, eventmask)

Modify a registered file descriptor.

epoll.unregister(fd)

Remove a registered file descriptor from the epoll object.

Changed in version 3.9:版本3.9中更改： The method no longer ignores the EBADF error.

epoll.poll(timeout=None, maxevents=- 1)

Wait for events. timeout in seconds (float)

Changed in version 3.5:版本3.5中更改： The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see PEP 475 for the rationale), instead of raising InterruptedError.

Polling Objects

The poll() system call, supported on most Unix systems, provides better scalability for network servers that service many, many clients at the same time. poll() scales better because the system call only requires listing the file descriptors of interest, while select() builds a bitmap, turns on bits for the fds of interest, and then afterward the whole bitmap has to be linearly scanned again. select() is O(highest file descriptor), while poll() is O(number of file descriptors).

poll.register(fd[, eventmask])

Register a file descriptor with the polling object. Future calls to the poll() method will then check whether the file descriptor has any pending I/O events. fd can be either an integer, or an object with a fileno() method that returns an integer. File objects implement fileno(), so they can also be used as the argument.

eventmask is an optional bitmask describing the type of events you want to check for, and can be a combination of the constants POLLIN, POLLPRI, and POLLOUT, described in the table below. If not specified, the default value used will check for all 3 types of events.

Constant	Meaning
POLLIN	There is data to read
POLLPRI	There is urgent data to read
POLLOUT	Ready for output: writing will not block
POLLERR	Error condition of some sort
POLLHUP	Hung up
POLLRDHUP	Stream socket peer closed connection, or shut down writing half of connection
POLLNVAL	Invalid request: descriptor not open

Registering a file descriptor that’s already registered is not an error, and has the same effect as registering the descriptor exactly once.

poll.modify(fd, eventmask)

Modifies an already registered fd. This has the same effect as register(fd, eventmask). Attempting to modify a file descriptor that was never registered causes an OSError exception with errno ENOENT to be raised.

poll.unregister(fd)

Remove a file descriptor being tracked by a polling object. Just like the register() method, fd can be an integer or an object with a fileno() method that returns an integer.

Attempting to remove a file descriptor that was never registered causes a KeyError exception to be raised.

poll.poll([timeout])

Polls the set of registered file descriptors, and returns a possibly-empty list containing (fd, event) 2-tuples for the descriptors that have events or errors to report. fd is the file descriptor, and event is a bitmask with bits set for the reported events for that descriptor — POLLIN for waiting input, POLLOUT to indicate that the descriptor can be written to, and so forth. An empty list indicates that the call timed out and no file descriptors had any events to report. If timeout is given, it specifies the length of time in milliseconds which the system will wait for events before returning. If timeout is omitted, negative, or None, the call will block until there is an event for this poll object.

Changed in version 3.5:版本3.5中更改： The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see PEP 475 for the rationale), instead of raising InterruptedError.

Kqueue Objects

kqueue.close()

Close the control file descriptor of the kqueue object.

kqueue.closed

True if the kqueue object is closed.

kqueue.fileno()

Return the file descriptor number of the control fd.

kqueue.fromfd(fd)

Create a kqueue object from a given file descriptor.

kqueue.control(changelist, max_events[, timeout]) → eventlist

Low level interface to kevent

• changelist must be an iterable of kevent objects or None

• max_events must be 0 or a positive integer

• timeout in seconds (floats possible); the default is None, to wait forever

Changed in version 3.5:版本3.5中更改： The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see PEP 475 for the rationale), instead of raising InterruptedError.

Kevent Objects

https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2

kevent.ident

Value used to identify the event. The interpretation depends on the filter but it’s usually the file descriptor. In the constructor ident can either be an int or an object with a fileno() method. kevent stores the integer internally.

kevent.filter

Name of the kernel filter.

Constant	Meaning
KQ_FILTER_READ	Takes a descriptor and returns whenever there is data available to read
KQ_FILTER_WRITE	Takes a descriptor and returns whenever there is data available to write
KQ_FILTER_AIO	AIO requests
KQ_FILTER_VNODE	Returns when one or more of the requested events watched in fflag occurs
KQ_FILTER_PROC	Watch for events on a process id
KQ_FILTER_NETDEV	Watch for events on a network device [not available on macOS]
KQ_FILTER_SIGNAL	Returns whenever the watched signal is delivered to the process
KQ_FILTER_TIMER	Establishes an arbitrary timer

kevent.flags

Filter action.

Constant	Meaning
KQ_EV_ADD	Adds or modifies an event
KQ_EV_DELETE	Removes an event from the queue
KQ_EV_ENABLE	Permitscontrol() to returns the event
KQ_EV_DISABLE	Disablesevent
KQ_EV_ONESHOT	Removes event after first occurrence
KQ_EV_CLEAR	Reset the state after an event is retrieved
KQ_EV_SYSFLAGS	internal event
KQ_EV_FLAG1	internal event
KQ_EV_EOF	Filter specific EOF condition
KQ_EV_ERROR	See return values

kevent.fflags

Filter specific flags.

KQ_FILTER_READ and KQ_FILTER_WRITE filter flags:

Constant	Meaning
KQ_NOTE_LOWAT	low water mark of a socket buffer

KQ_FILTER_VNODE filter flags:

Constant	Meaning
KQ_NOTE_DELETE	unlink() was called
KQ_NOTE_WRITE	a write occurred
KQ_NOTE_EXTEND	the file was extended
KQ_NOTE_ATTRIB	an attribute was changed
KQ_NOTE_LINK	the link count has changed
KQ_NOTE_RENAME	the file was renamed
KQ_NOTE_REVOKE	access to the file was revoked

KQ_FILTER_PROC filter flags:

Constant	Meaning
KQ_NOTE_EXIT	the process has exited
KQ_NOTE_FORK	the process has called fork()
KQ_NOTE_EXEC	the process has executed a new process
KQ_NOTE_PCTRLMASK	internal filter flag
KQ_NOTE_PDATAMASK	internal filter flag
KQ_NOTE_TRACK	follow a process across fork()
KQ_NOTE_CHILD	returned on the child process for NOTE_TRACK
KQ_NOTE_TRACKERR	unable to attach to a child

KQ_FILTER_NETDEV filter flags (not available on macOS):

Constant	Meaning
KQ_NOTE_LINKUP	link is up
KQ_NOTE_LINKDOWN	link is down
KQ_NOTE_LINKINV	link state is invalid

kevent.data

Filter specific data.

kevent.udata

User defined value.